- age under 18 years, elderly patients(>,=65 yrs), pregnancy, breast feeding, patient with bowel obstruction, and other structural intestinal disorders, gut hypomotility(chronic constipation(< 2 bowel movement/week for 1yr), hypomotility syndrome, megacolon, idiopathic pseudoobstruction), renal failure, congestive heart failure, liver failure, pre-existing electrolyte disturbance(hypercalcemia, hyperphosphatemia); refusal of consent to participate in the study.